1. Field of the Invention
This invention relates in general to a directed energy (“DE”) laser apparatus, and in particular to a high energy laser (“HEL”) apparatus involving an incoherent beam combining configuration.
2. Description of the Related Art
Recent advances in fiber lasers have made them a leading candidate for DE applications. Important characteristics of high-power fiber lasers include: i) high wall plug efficiency (>25%), ii) high CW power per fiber (˜2.5 kW), iii) single mode operation (TEM00), iv) good beam quality (M2<1.2), v) compactness (0.3 m3/kW), vi) satisfactory propagation wavelength (λ=1.075 μm), vii) relatively narrow linewidths (Δλ/λ˜0.1%) viii) low cooling requirements, ix) low maintenance, ix) long life (diode life>10,000 hrs), and xi) low operating cost.
To achieve the power levels needed for DE applications (>100 kW, CW), it is necessary to combine a large number of fiber lasers for efficient propagation over distances of many kilometers (>5 km). Fiber lasers can be combined spectrally, coherently, or incoherently. Coherent beam combining relies on constructive interference of many lasers to produce high intensities on a remote target. It requires precise phase locking of the fiber lasers, polarization matching, very narrow linewidths (δλ/λ<10−6) and good optical beam quality. These requirements are difficult to achieve in practice and limit the individual fiber laser power. The state-of-the-art in single-mode, single-fiber lasers having a well-defined polarization is <400 W. The propagation efficiency for coherent combining is also limited by the filling factor of the fiber array, i.e., a low filling factor results in a significant amount of optical energy in lobes outside of the central lobe. Spectral combining uses gratings to combine a large number of beams with slightly different wavelengths. This approach is limited by the fiber laser bandwidth and the requirement that the lasers have a well-defined polarization. To date, the highest total power achieved through coherent or spectral combining is less than 1 kW. Using currently available fiber lasers, a coherently or spectrally combined DE system would be complex and would require an extremely large number of lasers.